U1879
DTC U1879
Diagnostic Instructions
* Perform the Diagnostic System Check - Vehicle Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis Strategy Based Diagnosis for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions Diagnostic Procedure Instructions provides an overview of each diagnostic category.
DTC Descriptor
DTC U1879
- Drive Motor Control Module 2 Lost Communication with Engine Control Module
For symptom byte information, refer to Symptom Byte List Symptom Byte List.
Diagnostic Fault Information
Circuit/System Description
Devices connected to the serial data circuits monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the devices. The devices have prerecorded information about what messages are needed to be exchanged on the serial data circuits, for each virtual network. The messages are supervised and also, some periodic messages are used by the receiver device as an availability indication of the transmitter device. Each message contains the identification number of the transmitter device.
Conditions for Running the DTC
* The system voltage is greater than 9.5 V.
* The vehicle power mode master requires serial data communication to occur from this specific device.
Conditions for Setting the DTC
A supervised periodic message that includes the transmitter device availability has not been received.
Action Taken When the DTC Sets
The DTC U1879 is a type B DTC.
Conditions for Clearing the DTC
The DTC U1879 is a type B DTC.
Diagnostic Aids
* Sometimes, while diagnosing a specific customer concern or after a repair, you may notice a history U-code present. However, there is no associated "current" or "active" status. Loss-of- communication U-codes such as these can set for a variety of reasons. Many times, they are transparent to the vehicle operator and technician, and/or have no associated symptoms. Eventually, they will erase themselves automatically after a number of fault-free ignition cycles. This condition would most likely be attributed to one of these scenarios:
- A device on the data communication circuit was disconnected while the communication circuit is awake.
- Power to one or more devices was interrupted during diagnosis.
- A low battery condition was present, so some devices stop communicating when battery voltage drops below a certain threshold.
- Battery power was restored to the vehicle and devices on the communication circuit did not all re-initialize at the same time.
- If a loss-of-communication U-code appears in history for no apparent reason, it is most likely associated with one of the scenarios above. These are all temporary conditions and should never be interpreted as an intermittent fault, causing you to replace a part.
* A device may have a U-code stored in history that does not require any repairs. Issues with late or corrupted messages between devices can be temporary with no apparent symptom or complaint; this does not mean the device is faulty. Do not replace a device based only on a history U-code.
* Do not replace a device reporting a U-code. The U-code identifies which device needs to be diagnosed for a communication issue.
* Communication may be available between the device and the scan tool with one or more GMLAN serial data systems inoperative. This condition is due to the device using multiple GMLAN busses.
* Use Data Link References Initial Inspection and Diagnostic Overview to determine what GMLAN serial data communications the non communicating device uses.
* The device may not have internal protection for specific control circuits and may open a B+ or ignition fuse. If a fuse is open and the B+ or ignition circuit is not shorted to ground, ensure none of the control circuits are shorted to ground before replacing the non communicating device.
Reference Information
Schematic Reference
* Data Communication Schematics [1][2]Electrical Diagrams
* Control Module References Programming and Relearning
Connector End View Reference
Component Connector End Views Hybrid/EV Battery Pack X1
Description and Operation
Data Link Communications Description and Operation Description and Operation
Electrical Information Reference
* Circuit Testing Circuit Testing
* Connector Repairs Connector Repairs
* Testing for Intermittent Conditions and Poor Connections Testing for Intermittent Conditions and Poor Connections
* Wiring Repairs Wiring Repairs
Scan Tool Reference
Control Module References Programming and Relearning for scan tool information
Circuit/System Verification
1. Vehicle in Service Mode.
2. Verify that DTC U0100 is not set.
• If DTC U0100 is set
Refer to DTC U0100-U02FF U0100.
• If DTC U0100 is not set
3. Verify that DTC U0073, U0074, U0077, U0078, U007A, P06E4, P1EB9, U1814, U2099, B1325, B1330, B1370, B1380, B1424, B1428, B1440, B1441, B1517, C0800, C0899, C12E1, P0560, or P0562 is not set.
• If any of the DTCs are set
Refer to Diagnostic Trouble Code (DTC) List - Vehicle Diagnostic Trouble Code (DTC) List - Vehicle.
• If none of the DTCs are set
4. Refer to Circuit/System Testing.
Circuit/System Testing
Note: Use the schematics and connector end views to identify the device's ground, B+, ignition, accessory wake up serial data 2, and serial data circuit terminals.
1. Vehicle OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 meters (9.8 feet) away from vehicle. Disconnect the harness connectors at the K20 Engine Control Module. It may take up to 2 minutes for all vehicle systems to power down.
2. Test for less than 10 ohms between each ground circuit terminal and ground.
• If 10 ohms or greater
1. Vehicle OFF.
2. Test for less than 2 ohms in the ground circuit end to end.
• If 2 ohms or greater, repair the open/high resistance in the circuit.
• If less than 2 ohms, repair the open/high resistance in the ground connection.
• If less than 10 ohms
3. Vehicle in Service Mode.
4. If equipped, verify a test lamp illuminates between each B+ circuit terminal and ground.
• If the test lamp does not illuminate and the circuit fuse is good
1. Vehicle OFF.
2. Test for less than 2 ohms in the B+ circuit end to end.
• If 2 ohms or greater, repair the open/high resistance in the circuit.
• If less than 2 ohms, verify the fuse is not open and there is voltage at the fuse.
• If the test lamp does not illuminate and the circuit fuse is open
1. Vehicle OFF.
2. Test for infinite resistance between the B+ circuit and ground.
• If less than infinite resistance, repair the short to ground on the circuit.
• If infinite resistance, replace the K20 Engine Control Module.
• If the test lamp illuminates
5. Vehicle in Service Mode.
6. If equipped, verify a test lamp illuminates between each ignition circuit terminal and ground.
• If the test lamp does not illuminate and the circuit fuse is good
1. Vehicle OFF.
2. Test for less than 2 ohms in the ignition circuit end to end.
• If 2 ohms or greater, repair the open/high resistance in the circuit.
• If less than 2 ohms, verify the fuse is OK and there is voltage at the fuse.
• If the test lamp does not illuminate and the circuit fuse is open
1. Vehicle OFF.
2. Test for infinite resistance between the ignition circuit and ground.
• If less than infinite resistance, repair the short to ground on the circuit.
• If infinite resistance, replace the K20 Engine Control Module.
• If the test lamp illuminates
7. Vehicle in Service Mode.
8. If equipped, verify a test lamp illuminates between the accessory wakeup serial data 2 circuit terminal and ground.
• If the test lamp does not illuminate
1. Vehicle OFF, disconnect the harness connectors at the K114B Hybrid Powertrain Control Module 2.
2. Test for infinite resistance between the accessory wakeup serial data 2 circuit and ground.
• If less than infinite resistance, repair the short to ground on the circuit.
• If infinite resistance
3. Test for less than 2 ohms in the accessory wakeup serial data 2 circuit end to end.
• If 2 ohms or greater, repair the open/high resistance in the circuit.
• If less than 2 ohms, replace the K114B Hybrid Powertrain Control Module 2.
• If the test lamp illuminates
9. Vehicle OFF, all access doors closed, all vehicle systems OFF, and all keys at least 3 meters (9.8 feet) away from vehicle. Disconnect the X1 harness connector at the T6 Power Inverter Module. It may take up to 2 minutes for all vehicle systems to power down.
10. Test for less than 2 ohms in each serial data circuits end to end between the K20 Engine Control Module and the T6 Power Inverter Module.
• If 2 ohms or greater
Repair the open/high resistance in the serial data circuit.
• If less than 2 ohms
11. Replace the K20 Engine Control Module.
Repair Instructions
Perform the Diagnostic Repair Verification Verification Tests after completing the repair.
* GMLAN Wiring Repairs GMLAN Wiring Repairs
* Control Module References Programming and Relearning for device replacement, programming and setup